Given a Binary Tree, write a function to check whether the given Binary Tree is a prefect Binary Tree or not.
A Binary tree is Perfect Binary Tree in which all internal nodes have two children and all leaves are at same level.
The following tree is a perfect binary tree
10
/ \
20 30
/ \ / \
40 50 60 70
18
/ \
15 30
The following tree is not a perfect binary tree
1
/ \
2 3
\ / \
4 5 6